This invention relates to the measurement of magnetic fields produced by the human body, and, more particularly, to a biomagnetometer that permits automated correlation of magnetic field measurements with the source of the magnetic field within the body of the subject.
Organs within the human body, most notably the brain, the heart, and the nerves, generate or conduct electrical signals. These electrical signals in turn produce magnetic fields that may be measured by an instrument positioned externally to the body. Such an instrument, termed a biomagnetometer, includes a sensitive detector of very weak magnetic fields, since the magnetic field emitted by the brain, for example, is on the order of one-ten millionth of the magnitude of the earth's magnetic field.
As the technology of biomagnetometry advances, doctors and researchers are discovering correlations between the magnetic activity of the body and its state of health, and are also gaining an understanding of cognitive processes and other functions of the body. A primary application of the technology is deducing from the measured external magnetic fields the location and nature of the electric currents which were the sources of the fields. Both functional and dysfunctional portions of organs can be identified in this manner.
It is important that the magnetic field be measured correctly, and that the origin of the magnetic field be determined accurately from the measurement. The biomagnetometer must be capable of making accurate local magnetic field measurements external to the body and then utilizing those measurements in conjunction with geometric information about the body being measured and the measurement system to identify the operating electrical and magnetic field sources within the body and their location.
Local magnetic field measurements are made with biomagnetometers, which utilize sensitive detector systems having magnetic field pickup coils and Superconducting QUantum Interference Device (SQUID) detectors. Such systems, which are capable of measuring very small magnetic fields, also include sophisticated signal processing instrumentation that can isolate the measured magnetic field of interest from spurious fields produced by other sources and that are detected simultaneously. Such biomagnetometers are commercially available from Biomagnetic Technologies, Inc., San Diego, Calif.
An important advance in correlating the measured magnetic field with its precise origin in the body is disclosed in U.S. Pat. No. 4,793,355, which describes an electromagnetic transmitter/receiver system for automatically measuring the location of the body relative to the magnetic field detector. The position of the body is recorded along with the measured magnetic field. Thus, for example, the actual position of the head may be measured and continuously recorded at the same time as the magnetic fields produced by the brain are measured and recorded. If the subject's head moves slightly during the course of a magnetic field measurement, the correlation between magnetic field measurement and head position is still maintained. In prior approaches, the subject's head had to be restrained to a preestablished position during the measurements, and the restraint in itself could result in spurious magnetic field signals. A commercial version of the apparatus disclosed in U.S. Pat. No. 4,793,355 is also available from Biomagnetic Technologies, Inc., for use in conjunction with its biomagnetometers.
There remains the problem of correlating the measured magnetic field with the location and shape of its source organ within the body. An illustration drawn from the problem of most practical interest is helpful in understanding the significance of this problem. The brain is located within the skull of the subject. The thickness of the skull varies from location to location around the head, so that the brain is not located at some uniform depth below the surface of the skull. The existing biomagnetometers can make accurate measurements of the magnetic field produced by the brain at locations outside the skull, and can correlate those measurements to the skull location using the approach disclosed in the '355 patent.
However, the existing biomagnetometers cannot determine the shape of the interior surface of the skull and directly associate the measured magnetic field with a location within the brain--only to a location within the skull. Knowledge of the shape of the interior surface of the skull is also important in performing inverse calculations of the nature of the source from the external magnetic fields. Since there is an electrically conductive region that follows the interior surface of the skull, the skull can influence the measured external magnetic field produced by a source within the brain.
Knowledge of the location of the brain within the skull permits the physiological correlation of electrical signal and physical source that is the ultimate objective of the system. In the existing approaches, the shape of the interior surface of the skull and the location of the brain within the skull must be assumed from a model of the head, or provided by other measurements such as an X-ray of the head. After the magnetic field measurements are performed, an X-ray of the head can be used to correlate the position and shape of the brain with the position and shape of the skull. Such approaches are inaccurate, in the case of the assumption of a head structure, or imprecise and awkward to use, in the case of X-rays or the like.
There is a need for direct correlation of the position and shape of the source organ, such as the brain, and intervening electrically conductive structure, such as the skull, with the measured magnetic field. The greatest need is experienced in the area of brain measurements, because the magnitude of the magnetic fields is small and must be known accurately, because of the high required spatial accuracy of the correlations, and because of the presence of the skull around the brain. However, the need appears in relation to other biomagnetic measurements as well. The present invention fulfills this need, and further provides related advantages.